Rotary drill bit

ABSTRACT

A drill bit for drilling a borehole with a steerable drilling system, the drill bit comprising a longitudinal axis, and a cutting profile with a leading face section blended with a curved region into a tapered gauge region having a tapered gauge profile. The tapered gauge region being tapered with respect to the longitudinal axis determined by the steerable drilling system. The cutting profile and the gauge profile lie wholly within a bit profile envelope made up of a three dimensional surface of two complimentary conical sections set at the tilt angle and separated by and blended with a rounded section which forms an outermost diameter of the bit profile envelope. The rounded section has a diameter substantially equal to a maximum allowable API diameter for the drill bit, and further has a radius of curvature substantially equal to one-half the diameter.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a rotary drill bit, and in particular to animproved rotary drill bit suitable for use in a steerable drillingsystem.

2. Description of the Related Art

It is known, when drilling well bores for use in hydrocarbon extraction,to use steerable drilling systems to provide control over the directionin which the well bore is being drilled, and hence over the path alongwhich the well bore extends. One type of steerable drilling system usesa downhole motor to control the angular position of a housing relativeto the formation being drilled, the housing including an angled driveshaft carrying a drill bit such that the axis of rotation of the drillbit is angled to the axis of rotation of the housing. By appropriatecontrol over the speed at which the housing is rotated relative to thespeed at which the downhole assembly as a whole is rotated, the angularposition occupied by the housing, and hence the drilling direction canbe controlled. A steerable drilling system of this type falls into acategory of drilling systems known as point-the-bit systems. A number ofother steerable drilling systems are known.

Where a point-the-bit drilling system type is used with a conventionaldrill bit, upon using the tool to cause a change in drilling directionthe drill bit is tilted relative to the hole. Such tilting causes thebit to foul against the adjacent formation. In order for the desiredamount of tilting of the bit to occur, the parts of the formationagainst which the bit fouls must be drilled even though drilling ofthose parts of the formation may not otherwise have been necessary.Obviously this is undesirable as it is uneconomic and may result inincreased bit wear. It may also negatively impact upon the steeringaccuracy, and also cause an oversize borehole to be drilled. The presentinvention provides drill bits suitable for use in such applications inwhich changes in drilling direction can be achieved with reduced effort.

BRIEF SUMMARY OF THE INVENTION

According to the invention there is provided a drill bit for drilling aborehole with a steerable drilling system, the drill bit comprising alongitudinal axis, and a cutting profile comprising a leading facesection blended with a curved region into a tapered gauge region havinga tapered gauge profile, the tapered gauge region being tapered withrespect to the longitudinal axis at a tilt angle determined by thesteerable drilling system. The cutting profile and the gauge profile liewholly within a bit profile envelope comprising a three dimensionalsurface of two complimentary conical sections set at the tilt angle andseparated by and blended with a rounded section which forms an outermostdiameter of the bit profile envelope. The rounded section has a diametersubstantially equal to a maximum allowable API diameter for the drillbit, and further has a radius of curvature substantially equal toone-half the diameter.

The drill bit may comprise a bit body adapted to be rotated about anaxis of rotation, the bit having a leading face and a gauge region, theintersection between the leading face and the gauge region lying on anotional plane passing through the axis of rotation at a point aboutwhich, in use, the bit is tilted. Furthermore, the leading face of thebit may have a cutting profile of substantially part-spherical formhaving a center of curvature located at the said point, and the gaugeregion defines a gauge region of tapering shape.

The gauge region conveniently tapers at an angle at least as great asthe maximum angle through which the bit is tilted, in use. The taperedgauge region has an angle of taper with respect to the longitudinal axisof 8° or less, and more typically has an angle of taper with respect tothe longitudinal axis of 4° or less.

Such a bit is advantageous in that, upon tilting of the bit, fouling ofthe bit against the formation does not occur with the result that suchtilting can be achieved with relatively little effort.

The gauge profile is conveniently of frusto-conical shape.

The invention also relates to a steerable drilling system comprising apoint-the-bit type steering arrangement carrying a rotary drill bit ofthe type defined hereinbefore.

The steering arrangement conveniently includes a housing from which anangled drive shaft protrudes, the bit being mounted, in use, upon thedrive shaft, and a downhole motor arranged to drive the housing forrotation about its axis.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will further be described, by way of example, withreference to the accompanying drawings.

FIG. 1 is a diagrammatic view of a typical downhole steerable drillingsystem.

FIG. 1A is a diagrammatic view of a typical drilling rig for formingdirectionally drilled boreholes into the earth.

FIGS. 2 and 3 are diagrammatic views similar to FIG. 1 illustrating anembodiment of the invention.

FIG. 4 is a drill bit of the prior art, to help illustrate the benefitof the frusto-conical form of the present invention.

FIG. 5 is a diagrammatic view similar to FIG. 1 showing a drill bit ofthe preferred embodiment.

FIGS. 6A-6G are examples of seven of the many different bit profilesthat may fit within the drill bit profile envelope.

DETAILED DESCRIPTION OF THE INVENTION AND THE PREFERRED EMBODIMENT

FIG. 1A shows a drill string 2 suspended by a derrick 4 fordirectionally drilling a borehole 6 into the earth for mineralsexploration and recovery, and in particular petroleum. A bottom-holeassembly (BHA) 8 is located at the bottom of the borehole 6. Indirectional drilling, the BHA 8 typically has a downhole steerabledrilling system 9 (illustrated diagrammatically in more detail inFIG. 1) and comprises a drill bit 10 having a leading face 12 and agauge region 14. As the drill bit 10 rotates downhole it cuts into theearth allowing the drill string 2 to advance, forming the borehole 6. Inthe “point the bit” type of steerable drilling system 9 illustrated, thedrill bit 10 may be carried by a drive shaft 16 which passes through ahousing 18. Within the housing 18, the drive shaft 16 contains a bendsuch that the output part of the drive shaft 16 is not coaxial with thehousing 18, but rather is angled thereto.

The housing 18 is carried by a downhole motor 20 arranged to rotate thehousing 18 about its axis. The motor 20 is supported by the drill stringsuch that, in use, rotation of the drill string causes the motor 20,housing 18 and drill bit 10 to rotate, operation of the motor 20 causingthe housing 18 to rotate in the opposite direction with the result that,if the motor 20 is driven appropriately, the housing 18 can be heldagainst rotation while the drill string continues to rotate.

In use, during normal operation of the downhole steerable drillingsystem, the motor 20 is operated to hold the housing 18 against rotationfor a short period of time during which the drill bit 10 is rotated bythe drive shaft 16 about its axis to cause formation material to beabraded, gouged or otherwise removed. Periodically, the motor 20 iscontrolled to cause an adjustment in the angular position occupied bythe housing 18 which in turn causes an adjustment in the angle at whichthe drill bit 10 is held by the drive shaft 16.

It should be noted that similar problems are encountered in “push thebit” and other type of steerable drilling system 9. Although thesesystems may vary somewhat from details as described above, they all tendto cause similar problems with the drill bit 10, as the bit is typicallytilted at an angle 30 in the hole to effect a change in drillingdirection.

As illustrated in FIG. 1, adjustment of the angle 30 at which the bit 10is held causes the drill bit 10 to be tilted from its longitudinal axis32 about the point 22, for example to the position illustrated in brokenlines. In order to achieve this tilting movement of the drill bit theportions of the surrounding formation located within the shaded areas24, 26 must be removed to allow the drill bit 10 to occupy the newposition. Clearly the need to remove these parts of the formationresults in the system being wasteful, increasing the effort which isrequired in moving the drill bit 10 to this position, and this isundesirable.

In one embodiment of the invention, as illustrated in FIGS. 2 and 3, thedrill bit 10 is modified such that the leading face 12 thereof has acutting profile 28 of substantially part-spherical form. Additionally,the gauge region 14 is shaped to be of tapering form.

The cutting profile 28 which may be of part-spherical form has a centerof curvature located at the point 22. As a result, upon the drill bit 10being tilted about the point 22, no additional formation material needsto be removed to allow the leading face 12 of the drill bit 10 to bemoved. Further, as the gauge region is of tapering form, tiltingmovement of the drill bit 10 through an angle no greater than a designedmaximum tilt angle for the drill bit 10 does not bring the gauge region14 of the drill bit 10 into fouling engagement with the surroundingformations. It will be appreciated, therefore, that a drill bit of thetype illustrated in FIGS. 2 and 3 accommodates changes in drillingdirection while requiring only a relatively small amount of effort.

In practice, it is unusual for the change in angular movement of the bit10 to be greater than around 4°, however in some circumstances it may beas high as 8°. As such, the angle of taper of the gauge region 14(denoted as ‘A’ in FIG. 2) is conveniently made to match, or be slightlymore than, the angular movement (tilt angle 30), so as to ensure thatfouling of the bit 10 against the formation does not occur. However, theangle of taper ‘A’ of the gauge region 14 may be varied depending uponthe application and the amount of tilting of the drill bit which islikely to occur, in use.

In the illustrated embodiment, the leading face of the bit is ofhemispherical form and the gauge region 14 has a gauge profile 15 offrusto-conical form. The intersection between the hemispherical leadingface 12 and the frusto-conical gauge region 14 lies on a plane whichintersects the bit axis at the point 22. It will be appreciated that insuch an arrangement, a definite line exists at the intersection betweenthe leading face and the gauge region. Arrangements are possible inwhich the part-spherical face forming the leading face 12 mergessmoothly into the frusto-conical surface forming the majority of thegauge region 14. In such an arrangement, the gauge region 14 is not oftruly frusto-conical form as it includes a small a curved region 134 (inFIG. 5) which is preferably part-spherical in shape where it meets theleading face 112, as shown in the preferred embodiment of the presentinvention in FIG. 5.

In order to illustrate the benefit of the frusto-conical form as itincludes a small part-spherical region, a drill bit 210 of the prior artis shown in FIG. 4. As illustrated, the bit 210 has a bit body 211 and aperpendicular gauge region 214, leading to a face portion 212 and alongitudinal axis 232. The letter “D” represents the diameter of thisdrill bit 210 as it drills in a borehole 209. The dashed profile 200 ofdrill bit 210 represents its position within the borehole 209 when it istilted an angle 230 as discussed above. The dashed profile 200 of thedrill bit 210 has been shifted slightly to the left so that therightmost portion of the profile 202 is within and just touches theborehole 209. This has been done in order to clearly demonstrate thatthis bit, when tilted, now drills a borehole that is larger by “d1” thanthe intended gauge diameter “D”. As can be seen, this is caused in partby the straight (i.e. non frusto-conical gauge section) near the priorart bit's shoulder 204, and the displacement effect from shifting therightmost portion of the profile 202 drill bit 210 toward the left. Itis clear, therefore, that even if the frusto-conical form, as describedabove, were formed onto this drill bit 210, it would still drill aslightly oversized borehole. Although this effect may be quite minimalfor small, conventional tilt angles, up to say around 4° on relativelysmall diameter drill bits, it may become pronounced when the directionaldrilling application requires a tilt angle beyond this amount, and/or adrill bit diameter that is greater than about 8¾″. It is in theseinstances that the small curved, preferably part-spherical, region 134as shown in FIG. 5 becomes important.

The drill bit 110 of the preferred embodiment is illustrated in FIG. 5.Drill bit 110 includes a bit body 111 and the tapered gauge region 114having a gauge profile 115 with an angle of taper ‘A’ as describedabove, blended into the curved, preferably part-spherical, region 134,which is then blended with the profile portion 112 of the bit 110. Thereare a nearly infinite number of profile portion 112 designs possible asillustrated by an alternate profile portion 112A as shown. The smallcurved, preferably part-spherical, region 134 of the drill bit has anincluded angle 136 which is approximately double the tilt angle 30 (asshown in FIG. 3).

A drill bit profile envelope 138 may be defined for a bit once thediameter of the drill bit and amount of the tilt angle 30 have beenestablished. The outermost dimensions of the drill bit must fit withinthis bit profile envelope 138 to assure that it will not drill anoversize hole at the given tilt angle 30. The drill bit may be madesmaller than this envelope, in accordance with the American PetroleumInstitute's (API) specifications for the diameters of fixed cutter drillbits as defined in their Specification 7, Fortieth Edition, November2001, Effective date March 2002, section 9.2.1 “Diamond Bit Tolerances”,incorporated by reference herein.

The bit profile envelope 138 is a three dimensional surface comprisingthree sections, two complimentary tapered conical sections 142 and 144,inwardly tapered at their outermost ends at the tilt angle 30 andblended with a rounded section 140 which is a partial section of asphere of diameter 148. The diameter 148 is the outermost diameter 148of the bit profile envelope 138. The value for the diameter 148 is themaximum diameter for a given size of drill bit allowed by the abovereferenced API specification, and may be constructed by using themaximum allowable API bit diameter as the diameter 148 of the roundedsection 140. As stated above, since the rounded section 140 is a partialsection of a sphere of diameter 148, it has a radius of curvature 146equal to one-half of the maximum allowable API bit diameter. The roundedsection 140 is then blended with larger ends of the two inward taperedconical sections 142 and 144 and rotated about the longitudinal axis132. The taper section 142 in the bit profile envelope 138 is a mirrorof taper section 144. The height of the rounded section 140 is derivedfrom the radius of curvature 146 of this section 140 and the includedangle 136 of the drill bit, as described above.

As will be appreciated by those skilled in the art, a drill bit made tofit within the bit profile envelope 138 as described above will drill aborehole to the proper diameter even if it is operated through a rangeof tilt angles—all the way from 0° to the tilt angle 30 that is themaximum for the steerable drilling system 9.

There are nearly an unlimited number of different drill bit cuttingprofiles 28,128 available, each one suited suitable for a specificdrilling condition. However, most of these profiles may be adapted tofit within the bit profile envelope 138, as illustrated in FIGS. 6A-6G.

In FIG. 6A, the cutting profile 128 follows that taper, and terminatingin ‘active gauge’ cutting elements 150. FIG. 6B shows a gauge sectionwith a concave shape and may or may not have the same type of ‘activegauge’ cutting elements 150. FIG. 6C shows a concave gauge sectiondesigns to cut or ream the formation with very low protrusion gaugereaming cutters 152. FIG. 6D shows a concave gauge section similar toFIG. 6C, except it cuts or reams the formation with protruding gaugecutters 154.

In FIG. 6E the section 156 between the ‘active gauge’ cutting elements150 and the gauge cutters 158 is relieved from the bit profile envelope138. Finally in FIGS. 6F and 6G are two drill bit cutting profiles 128showing curves 160 (either circular or ellipse) with and without gaugereaming cutters 154, as described above

The drill bit 10, 110 may take a range of forms. For example, the drillbit 10, 110 may comprise a matrix-type bit body 11,111 into whichcutting elements, for example polycrystalline or single crystal diamondgrains are embedded or impregnated, the diamond material serving toabrade the formation material upon rotation of the drill bit 10, 110.Alternatively, rather than a matrix type bit body 11, 111 withimpregnated diamond grains, the bit body 11, 111 may be machined frommetal, preferably steel, and a series of cutting elements may be mountedupon the bit body 11, 111. Such cutting elements may take the form ofpolycrystalline diamond compact cutters in which a table ofpolycrystalline diamond is bonded to a substrate of less hard material,for example tungsten carbide, which, in turn, is mounted upon the bitbody 11,111. The bit body 11, 111 may be shaped to include a series ofupstanding blades upon which the cutters are mounted, channels beingformed between the blades. In such an arrangement, the bit body 11, 111may be arranged to include nozzles to allow drilling fluid to besupplied to the channels between the blades for the purposes of coolingand cleaning of the cutters and to carry away from the drill bitmaterial abraded, gouged or otherwise removed from the formation duringdrilling. In each case, in accordance with this embodiment of theinvention, the cutting profile 28, 128 of the leading face 12, 112 ofthe bit 10, 110 is of substantially part-spherical shape.

It will be appreciated that the specific arrangement describedhereinbefore may be modified in a number of ways within the scope of theinvention. For example, the system used to point the bit 10, 110 in thedesired direction may be modified somewhat. Further, a number of changesmay be made to the specific shape of the drill bit 10, 110 while stillensuring that fouling of the drill bit 10, 110 with the formation doesnot occur during tilting movement of the drill bit 10, 110. Othermodifications or alterations are also possible within the scope of theinvention.

Whereas the present invention has been described in particular relationto the drawings attached hereto, it should be understood that other andfurther modifications apart from those shown or suggested herein, may bemade within the scope and spirit of the present invention.

1. A drill bit adapted for drilling a borehole with a steerable drillingsystem, the drill bit comprising a longitudinal axis, and a cuttingprofile comprising a leading face section blended with a curved regioninto a tapered gauge region having a tapered gauge profile, the taperedgauge region being tapered with respect to the longitudinal axis to atleast a tilt angle determined by the steerable drilling system; whereinthe cutting profile and the gauge profile lie wholly within a bitprofile envelope comprising a three dimensional surface of twocomplimentary conical sections set at the tilt angle and separated byand blended with a rounded section which forms an outermost diameter ofthe bit profile envelope; the rounded section having a diametersubstantially equal to a maximum allowable API diameter for the drillbit, and further having a radius of curvature substantially equal toone-half the diameter.
 2. A drill bit according to claim 1, wherein thetapered gauge region is of generally frusto-conical form.
 3. A drill bitaccording to claim 2, wherein the tapered gauge region has an angle oftaper with respect to the longitudinal axis which is greater than thetilt angle through which the bit experiences, in use.
 4. A drill bitaccording to claim 3, wherein the tapered gauge region has an angle oftaper with respect to the longitudinal axis of 8° or less.
 5. A drillbit according to claim 4, wherein the tapered gauge region has an angleof taper with respect to the longitudinal axis of 4° or less.
 6. A drillbit according to claim 1, wherein the gauge region is of concave shape.7. A drill bit according to claim 1, wherein the gauge region is ofsmoothly curved convex shape.
 8. A drill bit according to claim 6,wherein the gauge region is relieved from the bit profile envelope.
 9. Adrill bit according to claim 1, wherein the gauge region is convexlycurved.
 10. A drill bit according to claim 1, wherein the gauge regionis free of cutting elements.
 11. A drill bit according to claim 1,wherein at least one cutting element is provided on the gauge region soas to form an active gauge.
 12. A drill bit according to claim 1,wherein at least one low protrusion reaming cutting element is providedon the gauge region.
 13. A drill bit according to claim 1, wherein theleading face section of the bit is of generally part spherical shape.14. A steerable drilling system comprising a point-the-bit type steeringarrangement having a tilt angle and carrying a drill bit comprising alongitudinal axis, and a cutting profile comprising a leading facesection blended with a curved region into a tapered gauge region havinga tapered gauge profile, the tapered gauge region being tapered withrespect to the longitudinal axis determined by the steerable drillingsystem; wherein the cutting profile and the gauge profile lie whollywithin a bit profile envelope comprising a three dimensional surface oftwo complimentary conical sections set at the tilt angle and separatedby and blended with a rounded section which forms an outermost diameterof the bit profile envelope; the rounded section having a diametersubstantially equal to a maximum allowable API diameter for the drillbit, and further having a radius of curvature substantially equal toone-half the diameter.
 15. A drilling system according to claim 14,wherein the steering arrangement includes a housing from which an angleddrive shaft protrudes, the bit being mounted, in use, upon the driveshaft, and a downhole motor arranged to drive the housing for rotationabout its axis.